The image that appeared on Sony employees' computer screens in the initial cyber hack.ImgurA top FBI official told a congressional committee Wednesday that the Guardians of Peace hackers who invaded Sony Pictures' systems had an "extremely high" level of sophistication and used hacking tools that would have gone undetected at most businesses.

"The malware that was used would have slipped and gotten past 90 percent of the net defenses that are out there today in private industry and been a challenge to state governments," said Joseph M. Demarest Jr., assistant director of the FBI's cyber division.

Testifying at the Senate Banking Committee hearing called to discuss cybersecurity in the financial sector, Demarest was asked about the possibility that nation states like North Korea could have attacked Sony and be pursuing other American companies.

Demarest added that the FBI, Sony and the company it has hired to probe the attack, Mandiant, have worked closely together and that federal officials had teams at Sony within hours of the attack's discovery.